As a result of increased coal production to help solve some energy needs, there is an increased coal tar availability from the coke formation process, a coal related industry. Of the various coal tar components, phenol and cresols are the most important with phenol being the most valuable because of its use as a raw material and chemical intermediate in a wide variety of chemical products ranging from heat resistant phenolic resins useful in appliances and electrical components to epoxy resins, medicinals and synthetic fibers. The demand for phenol has, consequently, far outdistanced the ability to supply the material from coal tar and for this reason phenol is now predominantly produced by alternate synthetic methods.
Cresols are used in significantly much smaller quantities for areas such as: ortho cresol for herbicides; meta and para cresols for phenol-formaldehyde resins for molding compounds and adhesives; and tricresylphate production. Any large influx of cresol supply brought about by a potential increase in coal tar products would only serve to over-supply a less demanding market. Cresols are structurally very similar to phenol, differing only in alkyl substitution on an aromatic ring. Dealkylation of these type materials could lead to increased supply of the more valuable phenol and at the same time relieve a potentially oversupplied cresol market.
Accordingly, an object of this invention is to provide an improved process for the hydrodealkylation of aromatic compounds.
A further object of this invention is to provide an improved hydrodealkylation process wherein small amounts of nonrecyclable dehydroxylated products are obtained.
Other objects and aspects, as well as the several advantages of this invention will be apparent to one skilled in the art upon a reading of the specification and appended claims.
According to the invention, an improved process is provided for the hydrodealkylation of aromatic compounds by contacting at least one alkylsubstituted aromatic compound with hydrogen in the presence of a low sodium content chromia-alumina catalyst under dealkylation conditions of temperature and pressure such that small amounts of nonrecyclable dehydroxylated products, such as benzene, toluene and xylenes, are obtained.
More specifically, it has been found that carrying out the hydrodealkylation of substituted aromatic compounds in the presence of a low sodium content chromia-alumina catalyst at temperatures below about 850.degree. F. results in the production of small amounts of nonrecyclable dehydroxylated products and this is contrary to carrying out the process at higher temperatures.
In accordance with another embodiment of the invention, small amounts of nonrecyclable dehydroxylated products such as benzene, toluene and xylenes are obtained when cresols are subjected to hydrodealkylation at temperatures below 850.degree. F. in the presence of a low sodium content chromia-alumina catalyst.
Thus, according to the invention, mixed cresols (ortho, meta, para) are hydrodealkylated to phenols and xylenols under mild reaction conditions when passed over a low sodium-content chromia on alumina catalyst. Utilization of the catalyst herein described results in a small amounts of nonrecyclable products (e.g. benzene, toluene, xylenes) being produced. Therefore, the invention provides a low sodium chromia based catalyst capable of dealkylating alkyl-substituted hydroxyaromatics, such as cresols, in the presence of hydrogen to phenol and xylenols with essentially no formation of nonrecyclable products.
The catalyst used according to the invention is a low sodium content chromia-alumina composite.
The catalyst useful in this invention as well as two control catalyst systems are all commercially available materials based on varying amounts of chromia on alumina. They are activated by heating for 30 to 60 minutes in the presence of hydrogen at slightly above the temperature employed in the hydrodealkylation reaction, this activation being conducted in the same tubular reactor in which the hydrodealkylation takes place. The distinguishing features between the inventive catalyst is the low chromia content and the low sodium content, the latter being the more important. Thus, it is preferred that chromia on alumina catalysts useful in this invention are those having very low sodium content (probably in the form of sodium oxide), less than 0.02 wt. % and with a chromium content between 3 to 55 wt. % on alumina preferably 5 to 25 wt. %.
The inventive catalyst maintains high product selectivity with approximately the same percent conversion when operated for long-time periods such as 16 hours. Operations beyond this time can require catalyst regeneration in which case common methods known in the art such as those employing air/nitrogen mixtures at elevated temperatures are quite satisfactory. Since the inventive catalysts do not significantly coke at the lower operating temperatures described herein, it is preferred that regeneration be conducted by merely passing a hot vaporized hydrocarbon (i.e., toluene) over the catalyst. This operation can be carried out in situ and successfully removes any residual materials.
The aromatic feed subjected to hydrodealkylation according to the invention can be any substituted aromatic compound whether monocyclic or polycyclic having various substituents. More specifically, the feed can be any substituted aromatic having at least one hydroxy group attached to the aromatic ring and having the general formula ##STR1## wherein x is 1 to 3 and y is 1 to 5, and the sum of x and y is 2 to 6. R is any hydrocarbyl radical including alkyl, cycloalkyl, alkenyl, or cycloalkenyl radicals having from 1 to 6 carbon atoms. For example, materials to be used, but not limited to, can be cresols (ortho, meta, para-substituted), xylenols (2,3-; 2,4-; 2,5-; 2,6-; 3,4-), trimethylphenols, 4-(2-propenyl)phenol, 2-cyclohexylphenol, 4-cyclohexylphenol, 4-(3-cyclohexenyl)phenol, and the like, and mixtures thereof.
Solvents can be used if so desired and can be, for example, alcohols (e.g., methanol) or aromatic hydrocarbons, preferably benzene.
Hydrogen is co-mixed with the feed and should be in a slight molar excess, preferably about 1.5 moles of hydrogen to 1.0 moles of alkylated hydroxyaromatic (e.g., cresol). Hydrogen helps to prevent unwanted condensation reactions which can lead to coke formation.
The rate of hydrogen-alkyl substituted hydroxy aromatic fed through the reactor should be between about 0.5 and about 10, preferably about 1.0 volumes of feed per volume of catalyst employed. This is referred to as liquid hourly space velocity (LHSV).
The hydrodealkylation conditions employed will be such that a small amount of nonrecyclable dehydroxylated products are obtained and such conditions will include temperatures below about 850.degree. F. and pressures below about 750 psig. In general, the conditions of reactions described herein are as follows:
______________________________________ Broad Range Preferred Range ______________________________________ Temperature, F. 550-850 650-800 , C. 287-454 343-426 Pressure, psig 300-750 400-600 , MPa 2.07-5.17 2.76-4.14 ______________________________________
Any type of reactor, but preferably a tubular reactor of stainless steel (e.g. 316) construction, can be employed. The walls of the reactor should be free of material which will interfere with the catalyzed reaction described herein. The catalyst should be positioned near the middle of the reactor and can be preceeded and followed by a zone of non-catalytic material such as quartz chips. In the specific examples, the catalyst is placed in the reactor chamber with a bed of inert non-catalytic material above and below the catalyst zone and the temperature raised to the reaction temperature while hydrogen gas is passed through the tubular reactor. This serves to dry and activate the catalyst prior to reaction. The run begins by pressuring the pre-heated (50 C.) feed through a filter into a Lapp pump and into the top mixing portion of the reactor zone. A static "o" ring switch, is set about 100 psi, 0.689 MPa above the operating pressure of the system to protect the pump. Hydrogen is pressured through a Moore back-pressure regulator, heated and mixed with the feed just before entering the mixing head. The hydrogen-feed mixture is passed through the reactor and through a steam-jacketed condensor and Moore back-pressure regulator into a chilled receiver. The products can then be analyzed and later separated usually by distillation.